In the past, many attempts have been made to develop methods of nondestructive inspection of structural or tubular fluid transmission components of plants and facilities, as exemplified by U.S. Pat. Nos. 4,415,980 and 4,425,505. Many of these inspection methods required that the facility be shut down and at least partially disassembled in order to obtain useful data. Nevertheless some prior art methods attempted to perform nondestructive testing on such components while the components were in use, such as U.S. Pat. Nos. 3,006,251; 4,352,065; 4,467,654; and 4,680,470. Of these, U.S. Pat. Nos. 3,006,251 and 4,680,470 disclose use of penetrating radiation.
U.S. Pat. No. 3,006,251 discloses a device that scans pipe, using both penetrating radiation and electromagnetic fields, while being translated along the length of oil and gas field pipes. However, the disclosed device has serious disadvantages. The scanning was not performed in a non-contacting manner and, therefore, was inoperable on hot insulated pipe. The device was also dependent on maintaining a concentric position around the pipe so that the radiation gauge would not be off center. The radiation gauge also gave only one thickness measurement rather that complete multi-angle and cross-sectional measurements. No method is disclosed for monitoring the position of the gauge either longitudinally or rotationally about the object as it scanned the pipe.
Additionally, the disclosed device could not be opened and closed about the pipe under examination, thereby virtually requiring that the system would have to be shut down and partially disassembled in order to position the device about in-place pipe to begin its inspection.
U.S. Pat. No. 4,680,470 discloses another prior art apparatus and method which uses penetrating radiation to detect flaws in installed objects. However, the disclosed method was passive in that it relied on background radiation for the radiation source, thereby restricting its use to detecting major defects or flaws. In particular, the primary system disclosed is only operable on objects that contain radiation sources inside the object. An alternative system is disclosed where a radioactive liquid is placed on the outer surface of the object, however the usefulness of this method is limited to showing flaws on the outer surfaces of the object and is impracticable to use in non-nuclear facilities due to safety and public health concerns. Since the strength of the radiation is unknown, the system is only capable of detecting flaws such as cracks and pits which manifest immediate changes in a localized area. More gradual flaws are much more difficult to detect with a passive system. Additionally, the disclosed system only contemplates translating the disclosed radiation detector along the length of a pipe but fails to disclose any particular means for accomplishing such movement.